1. Field of the Invention
This invention relates to radar signal processing systems, and more particularly, to a novel method and system for eliminating clutter in a radar return or echo signal.
2. State of the Prior Art
It is common practice to remove clutter, e.g., reflections from the ground received by an airborne radar system, by means of various types of well-known canceller circuits. For example, one known manner of eliminating main beam clutter (MBC) is to use a fixed frequency rejection filter in the canceller circuit to reject the main beam clutter. A frequency mixing operation is normally performed on the radar return prior to coupling the return to the fixed frequency rejection filter to place the center of the MBC spectrum at the center frequency of the rejection filter. In the more traditional analog radar circuits, a variable frequency source such as voltage-controlled quartz crystal oscillator (VCO) is usually employed to provide the locally generated signal required in performing the mixing operation. The calibration of such an oscillator is quite critical, requiring special care in manufacturing and testing, occasional readjustment due to aging of parts and extremely well-regulated power supplies.
In more recent digital radar systems, digital techniques typically eliminate the need for a VCO of the type described. However, a considerable amount of digital hardware is required. The hardware must be capable of high operating speeds and this, coupled with the large amount of hardware, results in large power consumption. For example, closed loop control of a mixer is required in order that the system antenna output will always be at the filter frequency. Additional hardware is thus required for detection and formation of the desired closed-loop circuit. Moreover, this closed-loop circuitry and mixer may limit the dynamic range of the radar processor at its input.